1. Field of the Invention
The present invention relates to stable crystals of sodium bromite trihydrate (NaBrO.sub.2.3H.sub.2 O) useful as brominating or oxidizing agents, and also relates to a process for preparing stable NaBrO.sub.2.3H.sub.2 O crystals containing impurities such as NaBr.2H.sub.2 O, NaBrO.sub.3 and others from an aqueous alkaline solution of sodium bromite combining operations of concentration under a reduced pressure, separation and cooling.
2. Description of the Prior Art
Sodium bromite (NaBrO.sub.2) has been conventionally known as a characteristic oxidizing agent, in that the oxidizing capacity of sodium bromite lies between that of strong oxidizing agents such as Cl.sub.2, Br.sub.2, NaClO, KMnO.sub.4 and Na.sub.2 Cr.sub.2 O.sub.7 and that of weak oxidizing agents such as KBrO.sub.3 and NaClO.sub.3 and the sodium bromite itself turns into a nonpolluting substance NaBr after it has performed the oxidation reaction. This compound has also been known as a brominating agent. Sodium bromite can be prepared typically by the relatively complex method that appeared in Japanese Patent Publication No. Sho 39-25543, in which the compound is prepared from NaOH, Br.sub.2 and Cl.sub.2 as raw materials. Aqueous alkaline solutions of sodium bromite prepared in this way are used for removing sizing agents such as starch, PVA and CMC in textile fibers and for controlling slime in industrial waters. However, the effective available bromine that stands for the amount of effective components gradually decreased day by day when the compound is given in a form of an aqueous solution. Sometimes the solution causes troubles in actual uses. Therefore, development and supply of sodium bromite of high storage stability has been desired.
The mentioned aqueous alkaline solution of sodium bromite is the only commercially available sodium bromite. The solution contains NaBr and other salts in almost saturated concentrations together with NaOH as alkali, and the concentration of NaBrO.sub.2 is as low as about 0.5 mil/liter. Such a dilute solution is inconvenient to use. Further, specific properties of sodium bromite as oxidizing agent are not known and other latent properties remain unrevealed. However, if this compound could be supplied in a form of stable solid of relatively high purity, developments for novel uses will be expected other than for removing sizing agent in textile fibers and for slime controlling of industrial water.
Methods for stablizing sodium bromite so far proposed include a stabilizing method by partially removing other salts present in the solution (Japanese Patent Publication No. Sho 45-37135) and a stabilized storage method in which hydrobromite or its salts thereof is reacted with a polymer containing quaternary ammonium group (Japanese Laid-Open patent application Nos. Sho 56-78406 and Sho 56-104704). Products by the former, Patent Publication No. Sho 45-37135, cannot be stored for a long term with sufficient stability, and products by the latter methods, Laid-Open patent application Nos. Sho 56-78406 and Sho 56-104704, though provided with slightly improved stability, require additional treatments when they are used for conventional purposes. In short, neither of these can be satisfactory in practical uses as a stabilizing or stabilized storage method.
Namely, decomposition of sodium bromite is presumed to be generally accelerated by the presence of heavy metals. In fact, presence of a minute amount of nickel and chromium has been confirmed in industrially produced aqueous solutions of sodium bromite. Further, it is believed that the stability of NaBrO.sub.2 in an aqueous solution is lowered when the solution contains relatively large amounts of other salts such as NaBr and NaBrO.sub.3.
Therefore, if sodium bromite having highly improved stability can be produced in a solid state, wider and different applications will be open to such sodium bromite, used as oxidizing and brominating agents.
Other knowledges on the same subject can be found in French Pat. No. 1,216,216 and the additional Patent thereof No. 72718. They teach stabilization of highly purified NaBrO.sub.2. 3H.sub.2 O by dissolving it in an aqueous alkaline solution.
Investigations have been done by the present inventors on French Pat. No. 1,216,216 using an aqueous NaOH solution and high purity product (more than 95% of purity) of NaBrO.sub.2. 3H.sub.2 O was obtained by recrystallization method. Stability was tested on the NaBrO.sub.2. 3H.sub.2 O crystals on an aqueous alkaline solution thereof. The NaBrO.sub.2 in the aqueous alkaline solution gradually decomposed as time went on, but the rate of decomposition was slower than that of the original raw material sodium bromite in an aqueous alkaline solution. On the other hand, however, the purified crystals themselves were less stable than the original raw material. The process of decomposition reaction can be recognized by eyes because yellow NaBrO.sub.2 turns into white NaBr and NaBrO.sub.3 on decomposition. The decomposition proceeds rather quickly in a single crystal. Thus, it is observable with a large crystal of NaBrO.sub.2. 3H.sub.2 O having 5 to 10 mm longer diameter that the decomposition reaction proceeds from one end to another.
In conclusion, therefore, NaBrO.sub.2 produced by the above-mentioned process is unexpectedly more stable when it is in the form of an aqueous alkaline solution than NaBrO.sub.2. 3H.sub.2 O crystals, though the reason therefor is not clear. The process for preparing high purity NaBrO.sub.2. 3H.sub.2 O is laborious, but nevertheless the crystals thus obtained are less stable. Therefore, these prior arts have no specific advantages over the conventional arts of storing NaBrO.sub.2 in the form of solution.